The present invention concerns a method for purification of water containing non-polar compounds, dispersed or dissolved in water.
In this invention, non-polar compounds means molecules with predominantly hydrophobic features so that their affinity to water is small, but they can be present in water even with a very low solubility or may have entered the water phase through a prior treatment which has brought them there either in molecular form or dispersed in one way or another or as droplets. Typical examples of such compounds are alkanes, wax, aromatics and larger hydrocarbons. It also comprises fatty acids, alcohols, amines and other organic molecules which are not very water soluble when the length of their hydrocarbon chain is long. Many of these molecules will not be tolerable in a natural environment; hence they should not be in an effluent flow. Many of them may have been added to the water in one process or another, and should preferably be collected and recycled to the process instead of being released into the nature. It is cost saving as well as environmentally friendly to reuse them.
As an oil field matures, the oil will have to be produced together with an increasing amount of water. This water will contain a certain amount of oil, chemicals and salts originating from the production process and the equilibrium condition present prior to start of the production.
The oil companies aim at achieving a maximum degree of recovery within the financial frames given by the investment costs (CAPEX), operational cost (OPEX), oil price and tax demands. At the same time, legislational restrictions to reduced discharge of oil and chemicals related to the activity gets stricter. Common for the different purification technologies used today are their removal of oil down to a certain level below the 30 ppm-requirement of OSPAR (abbreviation for Oslo-Paris, an EU consolidated protocol for oil and gas recovery in the north-east Atlantic Ocean). The discharge from the Norwegian shelf is below 20 ppm, and is moving towards 10 ppm. This is by the Norwegian government accepted as “zero-discharge”, while awaiting the results of the ongoing reports from the oil companies to SFT, and in absence of better technology.
New requirements and stricter interpretation of the zero-discharge requirement are expected from 2008/2009. Current solutions are not approved for arctic waters. This implies a need for development of new and better technology in order to satisfy the legislational requirements and make the oil companies able to produce oil also in the future.
NO 321.097 describes equipment for treating water by drawing completely clean water from the contaminated raw-water. Hence the impurities remain in the mother liquor. This has been shown to be very efficient for salts and many different water soluble (hydrophilic) chemicals. Purification efficiencies better than 90% removal of impurities are achieved in one purification step, i.e. substantially better than 99% after two steps. A third purification step would be able to improve the result even further; however this will more rarely be necessary. The efficiency increases after each new purification step.
From tests so far, it is evident that the main source of impurities carried on with the purified water is impurities existing as dissolved in water which get deposited on the surface of the hydrate crystals. This problem may be reduced by making the crystals as spherical as possible and with a diameter as large as possible. The round shape is beneficial because the ratio of surface to volume is the most favorable, while the size is important because the forces restraining water between the crystal grains (the capillary forces) are lower with large spheres than with small.
It has been shown that the most non-polar, oily, hydrophobic compounds dispersed in the flow of water, will have larger affinity towards the surface of the hydrates than towards the mother liquor/the water they come from. When such compounds are present in the raw water, they will rather quickly be deposited on the surface of the hydrate particles rather than staying dissolved or dispersed in the water. In this case oil will follow with the hydrates and the method according to NO 321.097, the Ecowat-process, will not perform optimally.
In this case it could be more optimal to direct the process towards crystals with a larger specific surface. This may be controlled through pressure and temperature. When the temperature is reduced and/or the pressure increased, the crystals will turn into flatter structures which may have projecting arms (dendrites), and thereby get a substantially larger specific surface.